1. Field of the Invention
This invention relates to a process for improving a liquid-liquid extraction process in which--in a two-phase system comprising a liquid aqueous phase and a liquid organic phase with one of the two phases as starting liquid--one or more valuable materials are separated from impurities and, where several valuable materials are present, from one another in a multistage extraction apparatus, the valuable materials being transferred by extraction from the starting liquid to the other phase which becomes charged with the valuable materials and the charged phase is washed with at least one washing liquid corresponding to the starting liquid.
2. Description of The Related Art
Liquid-liquid extraction processes are generally used to obtain or recover valuable materials, such as for example ions, compounds or complexes containing metallic elements, and are carried out in extraction columns, more especially in pulsed columns, in mixer-settlers or in a battery of centrifugal extractors, etc. Liquid-liquid extraction processes are also used in particular in the reprocessing of burnt-up nuclear fuels and/or fertile materials. In this case, uranium alone and/or plutonium is/are extracted from an aqueous dissolver solution containing nitric acid into the organic solution as uranyl nitrate or plutonium(IV) nitrate using tributylphosphate (TBP). To this end, the two phases flow in countercurrent in the apparatus mentioned. The organic phase used is generally a 30% by volume solution of TBP in an aliphatic diluent or solvent which effectively extracts the uranium or even the plutonium from solutions of nitric acid or from other solutions. In most cases, half the countercurrent extractor, in which the extraction takes place, may be used for washing the organic phase previously. In this case, therefore, the beginning and end of an extractor are so defined that the organic phase--in the present case the extractant phase--is introduced at the beginning of the extractor, an aqueous solution of uranium and, possibly, plutonium is fed as feed solution into a central part of the extractor and an aqueous washing solution is introduced at the end of the extractor. The uranium and the plutonium are analogously re-extracted from the charged organic phase as feed solution, the charged organic phase being introduced into a central part of the extractor. The aqueous phase--in this case the extractant phase--is then introduced at the end of the extractor while fresh organic phase is introduced at the beginning of the extractor, washing out from the aqueous re-extract those ingredients of the solution which are not to be re-extracted with the uranium or plutonium.
In the extraction process, the concentration of uranium in the aqueous feed solution can vary within wide limits. However, a reasonably high uranium concentration (70-90 g/l) has hitherto been sought in the organic phase in order to suppress the extractability of substances to be separated off (i.e. fission products, neptunium, etc.). Where the concentration of uranium in the feed solution was low, for example 30-50 g/l, the extractor was operated on a so-called dilute flow scheme. With a so-called concentrated flow scheme, the aqueous phase contains from 200 to 430 g U/l. It is clear that the ratio of the flow rates of the organic phase and the aqueous phase has to be kept low (0.3-0.7) in the dilute flow scheme and high (2-6) in the concentrated flow scheme. It is an advantage of the concentrated flow scheme that the total throughput of the phases is distinctly lower, i.e. by a factor of 1.6 to 3.5, than in the dilute flow scheme. This enables smaller extraction apparatus and vessels to be used in a reprocessing plant and, hence, building costs for shielded rooms to be reduced. By contrast, it may be regarded as an advantage of the dilute flow scheme that the effectiveness of separation of the extraction process is better. For example, neptunium(IV) which is only moderately extractable, is extracted together with uranium(VI) in the concentrated flow scheme because, in view of the high phase flow ratio, the extraction factor of neptunium(IV) is greater than 1 (the extraction factor is defined as the product of the distribution coefficient and the phase flow ratio). If the organic phase charged with uranium remains unwashed or is washed with a sufficiently concentrated solution of nitric acid, the neptunium leaves the extractor in the organic phase together with the uranium. If, however, the charged organic phase is washed with a dilute solution of nitric acid for better decontamination of fission products, the neptunium accumulates in the extractor. In a dilute flow scheme, the extraction factor of uranium is high enough for a good extraction yield whereas the extraction factor of neptunium(IV) falls to below 1 and the neptunium(IV) remains predominantly in the aqueous raffinate.